Solar Thermoplasmonic Nanofurnaces and Method for Making and Using Same

ABSTRACT

A thermoplasmonic device includes a titanium film and a plurality of titanium nitride tube elements disposed on the titanium film. Each of the titanium nitride tube elements includes an open top and a titanium nitride bottom. Each of the titanium nitride tube elements has titanium nitride tubular middle portion that extends from the open top to the titanium nitride bottom.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.16/865,365 filed May 3, 2020, which in turn claims priority to U.S.Provisional Application No. 62/843,058, filed on May 3, 2019, the entiredisclosures of both of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under grant numberDMR-1506775 awarded by the National Science Foundation, and grant numberFA9550-17-1-0243 awarded by the Air Force Office of Scientific Research.The government has certain rights in the invention.

TECHNICAL FIELD

The present invention relates to nanometer devices that convert solarenergy into heat energy.

BACKGROUND

Heat generation and management are among the most critical energy issueson a global scale. The heating and cooling sector are responsible for52% of overall energy consumption, 10% of which is produced fromrenewables. Solar-thermal technologies account for only 7% of therenewable heat generation, highlighting the extreme need for thedevelopment of novel and efficient solar-to-heat energy conversiontechnologies.

Present solar thermal technologies, such as parabolic troughs and solartowers, operate in the temperature range 300-600° C. to generaterenewable electricity. Such high temperatures can only be attained byconcentrated solar powers of the order of 100-1000 Suns, which can, atpresent, only be achieved by large area solar plants (i.e., 50 hectaresfor a 50 MW implant) that in turn requires extremely high capital costs(i.e., US$400 million).

Solar thermal technologies hold promise, not only for generatingrenewable electricity, but also for the development of chemical implantsthat use the high-generated temperatures to catalyze sustainablechemical transformations, such carbon dioxide reduction, hydrogengeneration, and liquid fuels synthesis through the Fischer-Tropschprocess. However, high-temperature catalysis may only becomemarket-competitive through the development of compact, cost-effectivethin-film devices that need lower concentrated solar power (and thuscheaper optical components) to reach high operating temperatures.

Plasmonics or metal nano-optics, offers an unprecedented control overlight at the nanoscale and has stimulated both new fundamental researchand application concepts in applied optics, photochemistry andnanoelectronics. Thermoplasmonics, a subfield of plasmonics, utilizesthe optical losses associated with the dissipation of surface plasmonsthat are excited in metal nanostructures upon external lightillumination and uses these conducting structures as nanosources ofheat. In other words, thermoplasmonics technology utilizes the stronglylocalized temperature increase due to the decay of surface plasmons uponlight absorption in metal nanostructures.

Since the early 2000s, the heating of metal nanoparticles using lighthas found applications in photothermal cancer therapy and otherbiomedical areas. Since then, the usage of local heating andphotothermal effects have led to new diverse applications such asheterogeneous catalysis, cavitation, steam generation, desalination, anddistillation of liquid solvents. Nevertheless, the applicability ofnanoparticle-based systems is limited by their low stability at hightemperatures which eventually causes the nanoparticles to aggregate.

Recent advances in the development of thermoplasmonic thin film systemshave overcome some of the intrinsic limitations of nanoparticles andhave found applications in areas such as templated growth ofnanostructures, optical nanotweezers, heat-assisted magnetic recording,and energetic materials. However, the thermoplasmonic devices reportedto date use micrometer-sized patterns and focused laser excitation toachieve the desired conditions and temperatures, and are thereforeunsuitable for practical applications requiring large-scalesolar-to-heat energy conversion. However, photonic crystals are aninteresting exception because they can be fabricated on large areaswithout compromising their exceptional optical properties. Photoniccrystals have been exploited as efficient absorber/emitter forthermophotovoltaics and solar thermophotovoltaics.

SUMMARY OF THE DISCLOSURE

In embodiments described herein, at least some of the above-statedshortcomings are addressed by large-scale films made by refractory(high-temperature stable) subwavelength titanium nitride cylindricalcavities that act as plasmonic “nanofurnaces” capable of reachingtemperatures above six hundred degree Celsius under moderateconcentrated solar irradiation. In at least some embodiments, thedemonstrated nanofurnaces show near-unity solar absorption in thevisible and near infrared spectral ranges and a maximum thermoplasmonicsolar-to-heat conversion efficiency of 68%. The nanofurnaces enablecontrolling chemistry at the nanometer scale with zeptoliter volumeprecision as well as processes such as melting of inorganic deposits toproduce homogeneous conformal coatings. Solar refractory thermoplasmonicnanofurnaces open a way to more efficient solar-to-heat energyconversion for activation of heterogeneous catalytic processes,thermoelectrics technology, and thermophotovoltaic devices.

In one embodiment, a thermoplasmonic device includes a titanium film anda plurality of titanium nitride tube elements disposed on the titaniumfilm. Each of the titanium nitride tube elements includes an open topand a titanium nitride bottom. Each of the titanium nitride tubeelements has titanium nitride tubular middle portion that extends fromthe open top to the titanium nitride bottom.

At least one embodiment is a method of scalable, flexible fabrication ofsolar absorber devices. A method to fabricate solar absorbers devices onsquare-centimeter areas produces refractory thermoplasmonic titaniumnitride (TiN) nanostructures having size and period one order ofmagnitude lower than that typical of photonic crystals. Two-dimensional(2D) sub-wavelength cylindrical nanocavity arrays generate temperatureshigher than 600° C. under moderate concentrated solar power. Eachnanocavity mimics a nanofurnace (or nanoreactor) enabling thermallyinduced nanochemistry with zeptoliter volume precision. In other words,the 2D sub-wavelength cylindrical nanocavity arrays according to thepresent disclosure can perform as broadband absorbers capable ofconcentrating the dissipated optical power in zeptoliter volumes togenerate the high temperatures.

These nanofurnaces can also be used to induce the thermoplasmonicmelting and decomposition of an iron organometallic precursor and newC—C bond formation, ultimately leading to the deposition of afew-nanometer-thick conformal layer of crystalline hermatite inside thenanofurnace walls. Metal nanoparticle decorated TiN nanofurnacesaccording to the present disclosure can catalyze CO oxidation reactionat a solar-to-heat thermoplasmonic efficiency of up to 63%.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a system according to oneembodiment of the present disclosure.

FIG. 2 is a flowchart of steps according to a method of the presentdisclosure.

FIG. 3a is a scanning electron microscopy image of the nano-structureformed at a step of the present method.

FIG. 3b is another scanning electron microscopy image of thenano-structure formed at a step of the present method.

FIG. 4a is a high resolution electron microscopy image of a lamellae ofa TiN film fabricated using the method of FIG. 2.

FIG. 4b is another high resolution electron microscopy image of alamellae of a TiN film fabricated using the method of FIG. 2.

FIG. 4c is yet another high resolution electron microscopy image of alamellae of a TiN film fabricated using the method of FIG. 2.

FIG. 4d includes high angle annular dark field scanning TEM image andEDS elemental mapping for Ti and N of crystallites forming the bottom ofa TiN nanofurnace fabricated using the method of FIG. 2.

FIG. 5 is cross-sectional representation of an array of TiN nanofurnacearray with additional layers.

FIG. 6a is a graph of the absorption spectrum of a TiN nanofurnaceaccording to the present disclosure.

FIG. 6b is a perspective representation of a TiN nanofurnace arrayaccording to the present disclosure.

FIG. 6c is a series of maps showing the electric field distributionachieved by the TiN nanofurnaces of the present disclosure at threeexcitation wavelengths.

FIG. 6d is a graph of electron relaxation rate (Γg) and electronrelaxation time (τg), respectively, as a function of grain size.

FIG. 6e is another graph of electron relaxation rate (Γg) and electronrelaxation time (τg), respectively, as a function of grain size.

FIG. 7a is a diagram of an arrangement to detect temperature variationsin the TiN nanofurnace disclosed herein under solar radiation.

FIG. 7b is a graph of solar-induced heat generation of the TiNnanofurnace and other related structures.

FIG. 7c is an infrared camera image of the thermal gradient of a TiNnanofurnace under solar radiation.

FIG. 7d is a graph of heat generation as a function of polarization.

FIG. 7e is a graph of heat generation as a function of incidence angle.

FIG. 8 is flowchart of another method according to the presentdisclosure.

FIG. 9a is a cross-sectional representation of the method of FIG. 8.

FIG. 9b is a collection of high angle annular dark field scanning TEMimages and EDS elemental mapping images of the nanofurnace after themethod of FIG. 8.

FIG. 9c is a graph of surface composition of the TiN nanofurnace andrelated structures after the method of FIG. 8.

FIG. 10 is flowchart of another method according to the presentdisclosure.

FIG. 11a is a representation of an array of TiN nanofurnaces, aperspective cut-away view a single TiN nanofurnace, and an enlargeddetail view of a portion of a single TiN nanofurnace of the presentdisclosure for use in performing solar thermal catalysis.

FIG. 11b is a high angle annular dark field scanning TEM image and EDSelemental mapping image of the nanofurnace employed for solar thermalcatalysis.

FIG. 11e is a graph of catalytic activity for CO oxidation of the TiNnanofurnace in FIG. 11 a.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the disclosure is therebyintended. It is further understood that the present disclosure includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles disclosed herein aswould normally occur to one skilled in the art to which this disclosurepertains.

The present disclosure contemplates the fabrication of titanium nitride(TiN) nanofurnaces. TiN is a refractory (high temperature stable)material that exhibits metallic properties in the visible andnear-infrared ranges and CMOS compatibility, which makes TiN preferableas an alternative to traditional noble metals such as gold and silverfor use as a thermoplasmonic material. Plasmonic TiN nano-antennas havebeen suggested and demonstrated in interconnects, second-harmonicgeneration, heat-assisted magnetic recording, water evaporation,plasmon-enhanced photoelectrochemical water splitting, and as broadbandabsorbers for solar-thermophotovoltaics. Notably, recent investigationson temperature-dependent optical properties of plasmonic materials hasrevealed that at temperatures above 400° C. the figures of merit oflocalized surface plasmon resonances (LSPR) and propagating surfaceplasmon polaritons (SPP) in thin TiN films become nearly the same asthose of polycrystalline noble metals. The TiN nanofurnaces disclosedherein take advantage of these optical properties, along with theexceptional hardness, thermal structural stability, and chemicalinertness, of TiN to produce efficient and robust solar-thermal devices.A schematic representation of one embodiment of practical utilization ofthe present disclosure is shown in FIG. 1.

Fabrication of Plasmonic Nanofurnaces

According to one embodiment, the TiN nanofurnaces are fabricated througha multistep process in the flowchart of FIG. 2. Initially, in Step 110,Ti foils are cleaned in acetone, ethanol and deionized water solutionsunder sonication. The titanium foil can have a thickness of 0.1-1 mm,with a preferred thickness of 0.125 mm. Thereafter, in Step 112, the Tifoils are anodized to form TiO₂ nanocavities. The anodization of Step112 is carried out in a hot acidic electrolyte inside a two-electrodeelectrochemical cell by using a platinum foil as the counter electrodeand Ti as the working electrode. The electrolyte is a mixture of hothydrofluoric acid (HF) and phosphoric acid (H₃PO₄), with theconcentration of the HF in the H₃PO₄ being 3 Molarity (3 m HF) in apreferred embodiment. A DC voltage is applied across the working andcounter electrodes, with the DC voltage being in the range of 10-30 VDC,for 1-3 hours. In a specific embodiment, anodization is performed at 15VDC for two hours. The anodization process produces highly homogenousTiO₂ nanocavity arrays that self-organize into close-packed hexagonallattices, as shown in the micrograph shown in FIG. 3a . It is noted thatanodization is an economical and easily scalable technique that can beused to produce nanostructures over large areas. After anodization, instep 114, TiO₂ nanocavity samples are rinsed with ethanol and driedunder nitrogen stream. In Step 116, TiO₂ nanocavities are converted toTiN in an ammonia atmosphere. In certain embodiments, the nitridizationoccurs at a temperature of 500-700° C., preferably at 600° C., under anammonia flow of 5-10 mL/minute, preferably 7 mL/minute, for 20-30minutes, preferably 25 minutes.

It is noted that a crystalline TiO₂ sample can be prepared for areference by annealing the TiO₂ nanocavities at 450° C. for one hour inair, using rapid thermal annealing. In addition, a reference sample,referred to herein as ‘TiN flat’, can be fabricated by annealing TiO₂nanocavities at 900° C. under ammonia flow for 10 minutes to induce thecollapse of the nanofurnaces.

Upon nitridation, the TiO2 nanostructured films turn from pale blue todark green or violet depending on the nanofurnace length (in the rangeof 150-200 nm), suggesting the formation of plasmonic TiN. After ammoniatreatment, the TiO₂ nanocavities are fully nitridized to TiNnanofurnaces with an average diameter of 80 nm, length of 180 nm, wallthickness of about 20 nm and center-to-center distance of 100 mm, asshown in the micrograph of FIG. 3b , resulting in nanocontainers with˜750 zeptoliter volume. During the conversion of TiO₂ to TiN, a 42%shrinkage of crystal cell volume introduces a strong mechanical stressin the nanostructures, which is released upon the transformation of theclosed-packed TiO₂ nanocavities to the TiN cylindrical nanofurnaces withslightly separated walls.

FIGS. 4a-4c show high-resolution transmission electron microscopy(HRTEM) images obtained by energy dispersive x-ray spectroscopy (EDS)elemental mapping of a lamellae of TiN thin film produced according tothe method described above. These images of a single TiN nanofurnacereveal that nitridation introduces a certain degree of porosity, both onthe bottom and on the walls of the nanocontainers, which may influenceoptical losses of plasmonic devices prepared in this manner. Moreover,the superimposed titanium and nitrogen mapping shows their evendistribution at atomic resolution (FIG. 4c ), while the deconvolutedelemental maps (FIG. 4d ) highlight that TiN nanofurnaces are composedby an equal amount (50%) of Ti and N, which can be expected forstoichiometric TiN. Interestingly, micrographs taken at the bottom of asingle TiN nanofurnace highlight the polycrystalline nature of itswalls, which are formed by crystallites having an average size of 9 nm,as shown in FIG. 4d . This morphological feature may affect surfaceplasmon dissipation. FIG. 4d shows high angle annular dark field (HAADF)scanning TEM image and EDS elemental mapping for Ti and N ofcrystallites forming the bottom of a single TiN nanofurnace.

Further morphological analysis reveals that during nitridation, thegradient diffusion of ammonia through the solid surface of the samplesresults in the formation of multilayer films including pure cylindricalTiN nanofurnaces and a Ti₂N thermal layer with thickness of ˜1 μmsitting on top of a Ti substrate, as depicted in the diagram of FIG. 5.Moreover, exposing the TiN nanostructures to air can lead to a 1 nm TiO₂oxidation layer on the outer surface of the structure.

Optical Properties of the Plasmonic Nanofurnaces

An important feature to achieve efficient nanoscale heat generation isrelated to the optical response of the plasmonic nanofurnaces disclosedherein. Total transmission (T) and reflection (R) from plasmonicabsorbers on glass are usually measured experimentally and then theabsorption is retrieved by using 1-T-R. In this case, the use of anoptically thick 125-μm Ti foil hinders light transmission so absorptionis defined as 1-R (transmission is 0) for the purpose of analyzing theoptical properties of the nanofurnaces disclosed herein. FIGS. 6a-6eillustrate some of the optical properties of the TiN nanofurnacesgenerated in accordance with the exemplary method described herein. FIG.6a shows the experimental absorption spectrum of the refractory TiNnanofurnaces disclosed herein (solid line) measured at normal incidence.TiN nanofurnaces are broadband absorbers capturing AM 1.5 G solarspectrum (shaded area of the spectral graph) with near unity efficiency,showing above average absorption of 98% over the visible range (380-700nm) and reaching a maximum peak of 99% at 470 nm. The absorptivityslightly decreases to 95% over the wider wavelength range of 300-1100nm, while it significantly drops to 77% if the upper wavelength limit isextended to 1400 nm. It can be seen that the experimental results are ingood agreement with the simulation for normal light incidence,represented by the lower line of circles. A possible method to maintainlight absorption above 90% is to deposit a 20 nm layer of a dielectricmaterial, such as Al₂O₃ on the surface of the nanofurnace. As shown inFIG. 6a , the modified nanofurnace (represented by the upper line ofcircles) maintains the average absorption above 96% in the range300-2000 nm. The addition of this dielectric layer may also act as aprotective layer against TiN oxidation.

FIG. 6b shows a schematic of the nanofurnace array used to obtain thesimulated optical results. The array includes a plurality of TiNnanofurnaces on a Ti₂N—Ti substrate. Each nanofurnace is a closedcylindrical structure with inner radius I′_(m)=30 nm and outer radiusr_(out)=45 nm. The total height of the furnace is h₁=250 nm, while theinner height is h₂=165 nm. The nanofurnace array forms a hexagonallattice, with r1=√3a and r2=a, where a=165 nm. The substrate is amultilayer structure that consists of a Ti₂N layer with thickness 1 μm,sitting on top of a Ti substrate. FIG. 6c shows a simulated absolutevalue of electric field amplitude distribution for three differentexcitation wavelengths—λ₁=300 nm (pure cavity resonance), λ₂=785 nm(hybrid resonance) and λ₃=1500 nm (off-resonance) applied to thenanofurnace array, which illustrates how localization varies upon theexcitation of different optical modes. FIG. 6d shows the electronrelaxation rate (Γ_(g)) and FIG. 6e shows the electron relaxation time(τ_(g)) dependence on inverse grain size (1/D) calculated with theMayadas-Shatzkes model at different values of the electron reflectioncoefficient (R).

Broadband absorption occurs due to light coupling into resonant cavitymodes of the nanofurnace array. Resonant behavior is determined by thefundamental waveguide mode of the cylindrical TiN nanofurnaces,characterized by the corresponding cutoff wavelength (k). Radiation withwavelengths smaller than λ_(c)=1130 nm is effectively coupled intocavity modes and enhanced absorption occurs due to the prolongedinteraction time and material losses of TiN nanofurnace walls. Some ofthe excited modes are standard cavity modes (pure cavity resonance),while some of them are hybrid cavity and surface plasmon polariton (SPP)modes (hybrid resonance). Such coupling also contributes to theincreased absorption. The radiation with wavelengths larger than λ_(c)is forbidden from entering the nanofurnaces; hence, the maincontribution into the absorption spectrum occurs due to localizedsurface plasmon resonance (LSPR) (off-resonance condition).

The resonance excited at 300 nm corresponds to pure high order cavitymode, which is characterized by four nodes and two antinodes havingdifferent E-field intensity distributions. Excitation of the pure modesproduces efficient light-matter interaction, which leads to the highestabsorption in the visible range (99% at 470 nm). The combination of highdensity of cavity modes of nanofurnace array and the plasmonic nature ofTiN leads to broadband absorption across the visible and near-IR ranges,centered at 785 nm and with full-width-half-maximum of 700 nm. Theexceptionally high broadening of this resonance may be the result of theroughness and presence of voids in TiN nanofurnaces as shown in thecross-sectional HRTEM images (FIGS. 4a-c ). From electric field(E-field) distribution at 785 nm (FIG. 6c ) it can be seen that alongfirst order cavity mode, the SPP waves propagating along the verticalsidewalls of the nanofurnaces are excited. Otherwise, absorption at theoff-resonance conditions (λ₃=1500 nm) is due to LSPR on the corners ofthe nanofurnaces, which leads to significant reduction of theabsorption.

From this picture of the optical properties of the nanofurnaces of thepresent disclosure, certain strategies for tuning light-matterinteraction in these TiN nanofurnaces emerge. First, simulations ofoptical absorption cross section of a single TiN cylindrical waveguidereveal that increasing the nanofurnace radius or diameter results in anabsorption cross section enhancement and a shift of k toward longerwavelengths. One approach for modifying the nanofurnace diameter isthrough the implementation of an imprinting step prior Ti anodization.Another approach involves the electrolyte in the anodization step. Asdiscussed above, the electrolyte is a mixture of hydrofluoric acid andphosphoric acid (3 M HF in H₃PO₄). This HF concentration producesnanocavity diameters of about 80 nm. It has been found that the diameterof the TiO₂ nanocavity can be varied by changing the HF concentration inthe electrolyte. Lower HF concentrations, in the range of 1-2M, producenanocavity diameters greater than 80 nm, although with an inhomogeneousarrangement of nanotubes. Increasing the HF concentration to 4-5 Mproduces diameters less than 80 nm, although the resulting filmresembles an electropolished surface rather than the nanotubularstructures of the preferred concentration (3 M). It is noted thatincreasing the anodization voltage can lead to significant increases indiameter, in some cases up to 200-250 nm. In this instance, thestructure is arguably no longer a nanocavity and is therefore lessusable as a nanofurnace.

A second strategy contemplates depositing an ultrathin layer of adielectric inside the nanofurnaces, as discussed above for the case ofAl₂O₃. This method may influence the fundamental waveguide modes, SPPmodes and their hybridization, and can be easily implemented with theaid of physical vapor deposition techniques.

An additional strategy to manipulate the light-matter interactionincludes tuning the electron relaxation rate (F), which is a fundamentalparameter of surface plasmons quantifying the overall quality of theresonances and, at the same time, influencing surface plasmondissipation into heat. The electron relaxation rate is the dampingconstant in the Drude term of the complex dielectric permittivity, andit is inversely proportional to the intraband electron relaxation time(τ)—i.e., Γ=h τ⁻¹ where h is Planck's constant. The relaxation rate hascontributions from electron-electron scattering, electron-phononscattering, scattering at grain boundaries, impurity and defectsscattering, and surface roughness scattering. For thin films (i.e.thickness above 50 nm) it is commonly accepted that grain boundarieshave little effect on Drude damping of conducting materials because thegrain size is usually comparable or much larger than the electron meanfree path. This is the usual picture for the majority of plasmonic filmsfabricated by e-beam deposition. However, the TiN nanofurnace filmsaccording to the present disclosure are processed through a nitridationstep that produces polycrystalline films with grain size (6-20 nm),which is much lower that mean free path of TiN, i.e. ≈45 nm, and whichis tunable depending on the processing temperature. Within this grainsize regime, grain boundary scattering contribution to opticalproperties becomes relevant and relaxation times (τ_(g)), as well asrelaxation rates (Γ_(g)=τ_(g) ⁻¹), could be estimated by using thequantitative model introduced by Mayadas. In this model, grainboundaries are modelled as N parallel partially reflecting planeslocated perpendicular to the electric field E, and placed an averagerandom distance apart corresponding to the average grain diameter (D).These reflecting planes are identified with scattering potentials,inside of which electron scattering is described by a relaxation time(τ_(g)). Therefore, for this geometry, the solution of the linearizedBoltzmann equation, which uses the electron-transition rate computed byperturbation theory, applying Fuchs boundary conditions, read as:

$\begin{matrix}{\tau_{g}^{- 1} = {\tau_{0}^{- 1} + \frac{1.37v_{F}R}{D\left( {1 - R} \right)}}} & (1)\end{matrix}$

where τ₀ ¹=Γ₀ is the relaxation rate for infinite grain size, which isretrieved from fitting of experimental ellipsometry measurements at roomtemperature on single crystalline TiN thin films and is 230 meV,D_(F)=7×10⁵ m s⁻¹ is the Fermi velocity for TiN, and R is the electronreflection coefficient at grain boundaries. The grain boundaryreflection coefficient, R is often taken to be 0.5 as a firstapproximation, but values between 0.3 and 0.7 has been previouslyreported.

FIG. 6d is a graph of the values of Γ_(g) for the TiN nanofurnacesdisclosed herein as a function of grain size. This graph shows thatΓ_(g) is strongly influenced by grain scattering in the films disclosedherein. For D=6 nm, scattering at grain boundaries indeed induces Γg tobe almost two times (513 meV, R=0.3), four times (819 meV, R=0.5), andeight times (1772 meV, R=0.7) the reference value of TiN single crystal.The values found for TiN nanofurnaces are unusually high, especially ifcompared with Γ_(g) of gold nanostructures, which have been reported tobe between 30 and 90 meV for a crystal size range of 200-40 nm. Asexpected, the TiN nanofurnaces of the present disclosure have a veryshort electron relaxation time, as reflected in the graph of FIG. 6e .The value of τ_(g) is 4.6 fs (R=0.5) for D=6 nm, for instance is threetime less than typical values reported for silver and gold.

The unusually high values of Γ_(g) and low values of τ_(g) suggest, onthe one hand. that TiN nanofurnaces have high optical losses, and, onthe other hand, that heat generation through momentum dissipation ofconducting electrons is favorable in the TiN nanofurnaces of the presentdisclosure and may be controlled by tuning the grain size.

Solar-to-Heat Energy Conversion in the Thermoplasmonic Nanofurnaces

To assess the thermoplasmonic performance of the nanofurnaces disclosedherein, samples are excited at normal incidence and an infrared (IR)camera is placed on the back of nanofurnace films at a 30° angle (FIG.7a ) to detect temperature variations under solar irradiation. Eachmeasurement is performed in air and is taken once steady-statetemperature is reached and corrected by IR emissivity values. Thesolar-induced thermoplasmonic heat generation of the samples is shown inthe graph of FIG. 7b , as represented by the dashed line. (Thedatapoints above the dashed line represent the results of simulations ofthe TiN nanofurnaces). The graph shows temperature as a function ofsolar power expressed in Suns (1 Sun=100 mW cm²). As shown in the graph,the TiN nanofurnace of the present disclosure outperforms nanofurnacesproduced by TiO₂ nanocavities, Ti foil and TiN flat reference samples,which are represented by the circles below the dashed line.

FIG. 7c shows an experimental infrared camera image of the thermalgradient departing from the illuminated central spot for TiNnanofurnaces under 19 Suns. Two operational parameters to consider forsolar energy conversion devices are (i) the influence of thepolarization state of light, and (ii) the angle of incidence on theabsorption and heat generation of structural components. Thesecharacterizations were performed with a 785 nm laser at 105 mW. Asexpected, polarization-dependent excitation generates a temperature of40±1° C. at all linear polarization angles (FIG. 7d ), in agreement withthe absorption data. The nanofurnaces of the present disclosure, due totheir cylindrical symmetry, represent the ideal polarization-insensitivecase for solar-thermal devices. On the other hand, thermoplasmonic TiNnanofurnaces show a significant dependence on incidence angle (θ_(i))between 70° and 40° (FIG. 7e ), while showing almost constant responsefor angle <40° up to normal incidence.

In the specific experiment, when 19 Suns of illumination power impingesthe TiN nanofurnace film of the present disclosure, an extremely highsolar thermoplasmonic temperature of 613° C. is reached. The outstandingperformance of the TiN nanofurnaces disclosed herein is furtherhighlighted when it is recognized that similar temperatures may bereached only by using a laser with 10⁶ greater power density. Despitethe high thermal conductivity of the Ti substrate for the TiNnanofurnace disclosed herein, high temperatures above 600° C. weregenerated in the experiment. This thermal performance leaves room forimprovements in heat management by thermally isolating the TiNnanofurnaces with substrates that can sustain high temperatures but havelow thermal conductivity, such as quartz, glass wool and fiber/foamglass.

The TiN nanofurnaces disclosed herein show a power-dependent heating of32.55° C./Suns as opposed to a flat TiN film which shows only 14.51°C./Suns, Ti foil with a rate of 15.85 and a TiO₂ nanocavity with 19.95°C./Suns. Even if a modified slope is calculated that accounts forabsorbed light flux, the TiN nanofurnaces exhibit a heating rate of 37°C./Suns versus a 30° C./Suns rate for the TiN film, Ti foil and TiO₂nanocavity. The TiN nanofurnaces take 20-30 sec to reach steady statetemperature generation, thus presenting a heating/cooling rate of ˜25°C./sec and demonstrating an exceptional resistance to thermal stress.This efficient light-to-heat conversion rate is one of the keyproperties of the nanofurnace disclosed. This high rate is ensured bythe proper optical response of the structure (i.e., broadbandabsorption) and by the large mode overlap of the cavity modes with thehighly lossy plasmonic material. Specifically, the mode analysis of thecavity modes shows that most of the energy is absorbed inside the thickplasmonic walls of the TiN nanofurnace. This leads to high local heatgeneration rates, which are essential for photocatalytic reactions. Inthis regard, the TiN nanofurnaces disclosed herein are superior tophotonic crystal structures that are not capable of efficient local heatgeneration.

Interestingly, the maximum temperatures reached follow a lineardependence on TiN grain size, confirming the intimate relation amongelectron scattering at grain boundaries, dissipation of electronmomentum, and thermoplasmonic heat generation.

These experimental results on solar thermoplasmonic heating areconfirmed by finite-element heat transport simulations under differentexcitation wavelengths of solar spectrum. Dissipated power is determinedfor each wavelength (Q_(i)), while total dissipated power (Q_(tot)) isdetermined by integration of Q_(i) over broadband solar spectrum,according to the equation:

(x,y,z)=∫_(λ) _(min) ^(λ) ^(max) Q _(tot)(x,y,z)dλ  (2)

Afterwards, the total dissipated power is applied as a heat source tothe quasi-static heat transfer problem. The total dissipated powerinside a TiN nanofurnace decreases in the order Q1 (λ₁=300 nm)>Q2(λ₂=785 nm)>Q3 (λ₃=1500 nm) as expected from the E-field intensitydistribution maps (FIG. 6c ). For λ₁=300 nm, exciting the pure cavitymode, the power is dissipated all along the nanofurnace walls with asignificant amount of losses also generated inside the nanofurnace base.A similar situation is verified for the cavity—SPP hybrid resonances(λ₂=785 nm) that, however, induce nearly no dissipation in the base ofthe nanofurnace. Conversely, the third excitation at 1500 nm (LSPR mode)produces only marginal power dissipation spreading over the nanofurnaceedges.

When considering broadband solar illumination, the simulated generatedtemperatures in air (represented by the upper line of circles in FIG. 7b) for increasing solar power excitation that differ slightly from theexperimental values represented by the dashed line. However, at highersolar power the difference among experimental and simulated values isonly 50-70° C. The slight discrepancy between the simulated and theexperimental temperatures may reflect the mismatch between TiNnanofurnaces film size (2 cm×1.5 cm) and the diameter of the focusedlight beam (˜1 cm) during the experiments. Furthermore, we utilize aFresnel lens to focalize solar light, and this set-up typically providesa strong gradient in light focus mirrored by a gradient of generatedtemperatures, as shown in the IR thermal image of FIG. 7 c.

To expand on the evaluation of the solar-to-heat conversion efficiencyof the present nanofurnaces, the thermal losses associated withconvection and radiation channels are computed, and then thethermoplasmonic solar-to-heat conversion efficiency (η_(STP)) isestimated. Conduction losses are negligible because samples aresuspended through a metallic clamp positioned on the samples border,where metal foils are close to room temperature. Convection losses(Pconv) are given by

P _(conv) =hA(T−T ₀)  (3)

where h=10 W m⁻² K⁻¹ is the heat transfer coefficient, A=1 cm² is thesurface area of the nanofurnaces, T is the thermoplasmonic temperaturegenerated in the nanofurnaces, and T₀ is room temperature.

Radiative losses (P_(rad)) are computed by integrating the black bodyradiation spectrum at the temperature generated inside the nanofurnacesover wavelength by taking into account the spectral dependent emissivityfunction, and are given by

P _(rad) =A∫ _(λ) _(min) ^(∞)ε(λ)I _(BB)(λ,T)dλ  (4)

where ε(k) is the emissivity of the nanofurnace surface, and I_(BB) isthe black body irradiance at operational temperature, which reads as

$\begin{matrix}{{I_{BB}\left( {\lambda,T} \right)} = {\frac{2\pi{hc}^{2}}{\lambda^{2}}\frac{1}{{\exp\left( \frac{hc}{\lambda k_{B}T} \right)} - 1}}} & (5)\end{matrix}$

where h=6.626×10⁻³⁴ J s and k_(B)=1.381×10⁻²³ J K⁻¹ are the Planck andBoltzmann constants, respectively, and c=2.998×10⁸ m s⁻¹ is the speed oflight.

Using Kirchhoff's law, which states that emissivity of the surface isequal to absorption, the nanofurnace absorption spectrum is computed byusing the experimental TiN complex dielectric permittivity measured athigh temperatures, and by using simulated absorption values (emissivity)in the computation of P_(rad). Finally, the thermoplasmonicsolar-to-heat conversion efficiency (η_(STP)) is computed, which isgiven by

$\begin{matrix}{\eta_{STP} = {\frac{P_{in} - P_{conv} - P_{rad}}{P_{in}} \times 100}} & (6)\end{matrix}$

Therefore, for the case when TiN nanofurnaces operate in air at 15 Sunsirradiation (P_(in)=1.5 W for an area of 1 cm²) generating 520° C., thethermal losses correspond to P_(conv)=0.495 W and P_(rad)=0.480 W. Inthese conditions, η_(STP) (air)=35%, while ruling out convective heattransfer losses (i.e. vacuum conditions) gives η_(STP) (vacuum)=68%.

Interestingly, when the TiN nanofurnaces reach high temperatures in air,a surface TiO₂ layer readily forms and the proper working mechanism iscompromised, thus not sustaining the maximum temperature for a prolongedtime. In stark contrast, when operating the TiN nanofurnace under inertgas (Ar) or vacuum atmosphere, the bulk composition, surface propertiesand morphology are perfectly retained.

Conformal Hematite Deposition with the Thermoplasmonic Nanofurnaces

An exemplary usage of the nanofurnaces generated by the above describedmethods is shown in flowchart of FIG. 8. In general, the methodincludes, in Step 210 depositing a first material in at least a firsttitanium nitride nanofurnace disposed on a titanium film, the titaniumnitride nanofurnace having an open top, a titanium nitride bottom, and atitanium nitride tubular middle portion extending from the open top tothe titanium nitride bottom. As discussed below, the first material maybe organometallic material. The method then includes, in Step 212,applying light to the titanium nitride nanofurnace. In Step 214, thetitanium nitride nanofurnace uses the applied light to heat the portionof the first material therein to bring about at least chemical and/orphysical change in the portion of the first material.

In one example of the operations of FIG. 8, the thermoplasmonic meltingof an inorganic deposit under concentrated solar light (FIG. 9a ) iscarried out. The nanofurnaces films are immersed in a concentratedacetone solution of Fe^(III)(acac)₃ (iron acetylacetonate) for 12 hours,and then rinsed with acetone before drying under nitrogen flow. Thisprocessing step induces the precipitation of sub-micrometric particleson top of the nanofurnaces and irregular deposits inside thenanofurnaces, without affecting the average diameter of each aperture.Notably, TiN nanofurnaces generate high thermoplasmonic temperatures ofabove 600° C. under the illumination of 15 Suns in vacuum for four hours(at η_(STP)=68%), and melt down the Fe deposit, resulting in a10-nm-thick conformal coating. This feature demonstrates that thedeposit forms in plasmonic hot spots of TiN nanofurnaces, where heatdissipation is maximized. HAADF-STEM and EDS elemental mapping imagesevidence the atomic homogeneity of the overlayer deposition at asub-nanometer scale (FIG. 9b ). The micrographs reveal that (i) thenanofurnace walls are covered by an iron oxide layer (see the brighterarea in the Fe and O maps of FIG. 9b ); (ii) oxygen is mostlyconcentrated on the Fe layer; (iii) the nanofurnaces preserve theinitial Ti and N atomic distribution after four-hour illumination undervacuum conditions (as shown by the brighter area in the Ti—N combinedmap of FIG. 9b ). Rietveld refinements on XRD patterns did not showeither iron-containing phases or TiO₂.

Several areas on different samples before and after irradiation (FIG. 9c) are analyzed to identify the iron oxide phase deposited onto thesurface of TiN nanofurnace walls. Raman spectroscopy reveals that, whenthe nanofurnace is in OFF state (i.e., without irradiation), only thefingerprint of TiN is detected, as represented by the second line in thegraph. In contrast, after the nanofurnace has operated in the ON state(i.e., under light irradiation at 15 Suns), as represented by the upperline in the graph, sharp and well-defined peaks typical of hematite(α-Fe₂O₃) appear, along with an additional peak denoting the formationof anatase TiO₂, probably growing at the interface between TiN andα-Fe₂O₃. The black circles above the upper line denote hematite bands,while the asterisk highlights the bands assigned to anatase TiO₂.Furthermore, the Raman spectrum on nanofurnaces that have experiencedthe thermal cycle shows two very intense peaks due to the D and G bandsof graphitic materials, demonstrating that temperatures generated insidethe nanofurnaces induce the degradation of both the organometallic Feprecursor and the superficial adventitious carbon, catalyzing theformation of new C—C bonds.

These findings illustrate the capability of TiN nanofurnaces to generatehigh thermoplasmonic temperatures under concentrated solar light.Primarily, nanofurnaces produce the decomposition of an ironorganometallic precursor and drive the formation of new C—C bonds.Thereafter, the nanofurnaces are capable of melting and re-depositing aconformal layer of crystalline hematite.

Solar-Thermal Heterogeneous Catalysis Using the TiN Nanofurnace

Another exemplary usage of the nanofurnaces generated by the abovedescribed methods is shown in flowchart of FIG. 10. In general, themethod includes, in Step 310 flowing a first molecular gas in at least afirst titanium nitride nanofurnace disposed on a titanium film, thetitanium nitride nanofurnace having an open top, a titanium nitridebottom, and a titanium nitride tubular middle portion extending from theopen top to the titanium nitride bottom. As discussed below, the firstmolecular gas may be carbon monoxide. The method then includes, in Step312, applying light to the titanium nitride nanofurnace. In Step 314,the titanium nitride nanofurnace uses the applied light to heat thefirst molecular gas therein to bring about a chemical transformation ofthe first molecular gas.

In one example of the operations of FIG. 10, a model CO oxidationreaction promoted by different solar intensities can be used todemonstrate the use of the TiN nanofurnaces disclosed herein for solarthermal catalysis. FIG. 11a shows a TiN nanofurnace decorated withcatalytic nanoparticles for heterogeneous catalysis. Rh nanoparticlesare deposited by immersing a nanofurnace film in ultrapure watercontaining RhCl₃ as a metal precursor, which was then reduced to Rh⁰ bythe addition of an aqueous solution of ammonia borane. As a result, Rhnanoparticles with size of 3-5 nm are homogeneously distributed over theinterior surface of the TiN nanofurnaces film as shown in FIG. 11bdepicting the EDS elemental for Ti and Rh of a single TiN nanofurnace.In the example, the Rh nanoparticles were deposited both on the mouthand on the inner part of the thermoplasmonic cavity.

The prepared TiN/Rh nanofurnaces were tested in the CO oxidation to CO₂by generating different temperatures at varying light intensities, asshown in the graph of FIG. 11e . In dark conditions, appreciable amountof CO₂ was observed. However, during solar irradiation the CO₂ formationrate increased rapidly and reached a plateau of ˜16 mol h⁻¹ m⁻² of CO₂at ˜9 Suns irradiation and at a generated temperature of ˜325° C. At thesame time, a stoichiometric O₂ conversion was observed. The conversionactivity for CO oxidation was always in the range 93-95%.

Notably, the light intensity dependence of the CO₂ generation ratefollows the sigmoidal shape typical of thermally activated catalyticprocesses. The thermoplasmonic TiN nanofurnaces activated Rhnanoparticles that catalyzed the CO oxidation, with naked TiNnanofurnaces that did not show any significant activity, i.e., 0.24 molh⁻¹ m⁻² at 15 Suns irradiation generating a temperature above 500° C.The η_(STP) of the nanofurnaces during the CO oxidation was determinedunder the conditions at which the catalytic conversion rate reached 50%of the final value, i.e., light intensity of 6.7 Suns and temperature of235° C. When the TiN nanofurnaces operate in air under 6.7 Sunsirradiation and generate a temperature of 235° C., the thermal lossesare P_(conv)=0.21 W and P_(rad)=0.037 W. Thus, η_(STP) (air)=63%, andwhen excluding P_(conv) (which do not occur in vacuum) η_(STP)(vacuum)=94.5%. Considering that the CO gas molecules may affect theconvection losses similarly to air, the TiN/Rh nanofurnaces catalyzed COoxidation at a solar-to-heat conversion efficiency of 63%.

In order to evaluate the nanofurnace stability, TiN nanofurnaces weretested in CO oxidation after a treatment with an accelerated agingprotocol under 15 Suns irradiation and flowing CO and O₂. The catalyticconversion rate reached 50% of the final value at a light intensity of8.4 Suns and a temperature of 291° C., thus showing a partialdeactivation with respect to the pristine sample and likely associatedto the beginning of TiN oxidation as suggested by XPS analysis. Notably,if the TiN nanofurnaces were treated, instead, with an accelerated agingprotocol under Ar, they showed very minor structural modifications,suggesting their higher stability for reaction performed in reducingconditions such as the challenging and environmentally relevanthydrogenation of carbon dioxide and ammonia synthesis.

The present disclosure should be considered as illustrative and notrestrictive in character. It is understood that only certain embodimentshave been presented and that all changes, modifications and furtherapplications that come within the spirit of the disclosure are desiredto be protected. Further details of the experimental embodiments,results of those experiments and the physical and chemical properties ofthe TiN nanofurnaces fabricated and evaluated in those experiments areincluded in Appendices A and B accompanying this application. Theentirety of both Appendices A and B are incorporated herein byreference.

1. A thermoplasmonic device, comprising: a titanium film; a plurality oftitanium nitride tube elements disposed on the titanium film, each ofthe titanium nitride tube elements having an open top, a titaniumnitride bottom, and a titanium nitride tubular middle portion extendingfrom the open top to the titanium nitride bottom.
 2. The thermoplasmonicdevice of claim 1, wherein at least some of the plurality of titaniumnitride tube elements have a length from the open top to the titaniumnitride bottom of 150 nm to 200 nm.
 3. The thermoplasmonic device ofclaim 2, wherein at least one of the plurality of titanium nitride tubeelements has a diameter of approximately 80 nm.
 4. The thermoplasmonicdevice of claim 3, wherein the at least one of the plurality of titaniumnitride tube elements has a wall thickness of approximately 20 nm. 5.The thermoplasmonic device of claim 3, wherein the center-to-centerdistance between at least two adjacent titanium nitride tube elements isapproximately 100 nm.
 6. The thermoplasmonic device of claim 1, whereinat least some of the plurality of titanium nitride tube elements have adiameter of approximately 80 nm.
 7. The thermoplasmonic device of claim1, wherein at least some of the plurality of titanium nitride tubeelements have a wall thickness of approximately 20 nm.
 8. Thethermoplasmonic device of claim 1, wherein the center-to-center distancebetween at least two adjacent titanium nitride tube elements isapproximately 100 nm.
 9. The thermoplasmonic device of claim 15, furthercomprising a conformal layer of crystalline hermatite inside at leastone of the titanium nitride tube elements.
 10. A thermoplasmonic device,comprising: a titanium film; a plurality of TiN tube elements disposedon the titanium film, each of the TiN tube elements having an open top,a TiN bottom, and a TiN tubular middle portion extending from the opentop to the TiN bottom; and a Ti₂N later disposed between the titaniumfilm and the plurality of titanium nitride tube elements.
 11. Thethermoplasmonic device of claim 10, wherein at least some of theplurality of titanium nitride tube elements have a length from the opentop to the titanium nitride bottom of 150 nm to 200 nm.
 12. Thethermoplasmonic device of claim 10, wherein at least some of theplurality of titanium nitride tube elements have a diameter ofapproximately 80 nm.
 13. The thermoplasmonic device of claim 10, whereinat least some of the plurality of titanium nitride tube elements have awall thickness of approximately 20 nm.
 14. The thermoplasmonic device ofclaim 10, wherein the center-to-center distance between at least twoadjacent titanium nitride tube elements is approximately 100 nm.
 15. Thethermoplasmonic device of claim 10, further comprising a plurality ofcrystallites formed on the bottom of at least one of the plurality of atitanium nitride elements.
 16. A thermoplasmonic device, comprising: atitanium film; a titanium film; a plurality of titanium nitride tubeelements disposed on the titanium film, each of the titanium nitridetube elements having an open top, a titanium nitride bottom, a titaniumnitride tubular middle portion extending from the open top to thetitanium nitride bottom, and a plurality of crystallites formed on thetitanium nitride bottom.
 17. The thermoplasmonic device of claim 16,where at least some of the plurality of crystallites has a diameter ofapproximately 9 nm.
 18. The thermoplasmonic device of claim 16, furthercomprising a conformal layer of crystalline hermatite inside at leastone of the titanium nitride tube elements.
 19. The thermoplasmonicdevice of claim 16, wherein at least some of the plurality of titaniumnitride tube elements have a length from the open top to the titaniumnitride bottom of 150 nm to 200 nm.
 20. The thermoplasmonic device ofclaim 19, wherein at least one of the plurality of titanium nitride tubeelements has a diameter of approximately 80 nm.